The heat with higher temperature is more useful and valuable. In thermodynamics, the heat at high temperature is called high-quality energy. These energy sources, such as pentane (C5H12), gasoline, hydrogen, electricity, friction energy etc, have a common feature in reaching an extremely high temperature when applied. On the contrary, the thermal energy at low temperature is called low-quality energy or waste heat which is of limited use. For example, the heat of the cooling water recycled by a cooling tower of a power plant can be used for domestic hot water only. A conventional solar water heater generates hot water at less than 100° C., which can be used for a warmer, but cannot for generating electricity, cooking or refrigerating.
To make a wider range of application of solar energy, it is necessary to improve heat collecting temperature. There are five types of zones in China in view of the distribution of solar energy. Since the energy density of solar rays is low, it is necessary to increase heat collecting temperature by light convergence, namely, to converge a large area of solar rays at a very small area. The bigger the concentrating ratio is, the higher the working temperature of the heat collector is. The concentrating ratio is generally 3-10 times, and the working temperature can reach 200° C.-400° C. If the concentrating ratio is increased to 1000-3000 times, the working temperature can be higher, which is upto above 1000° C.-3000° C.
Generally, there are three types of convergence of sunlight: (1) Multi-glass reflection focus. For example, in early 1970s, a giant solar furnace with above 40 meters tall was built at Odeillo in Pyrenees of France, which has a focus temperature up to 3500° C. and an output power up to 1000 kw. It is constituted by more than 11000 flat mirrors and automatically tracks the sun. As another example, the electric station Sun No. 1 of State of California in the United States has an output up to 10,000 kw and a working temperature up to 500° C. It is constituted by 1818 pieces of heliostats, each of which is 390.1 m2, and has a total area of 71064 m2 and a height of 55 meters. The shape is very spectacular. (2) Parabolic concave mirror reflection focus (solar oven). (3) Groove focus (line focus). All the methods described above belong to reflection focus which is the only way of convergence of solar energy so far.
Sunlight is reflected to a point of tower by all the flat-mirrors which track the solar movement to ensure that the sunlight concentrate on a focal point. The tracking of the reflecting mirrors needs an adjustment in two directions (double-axis), so as to track solar movement and changes of the solar elevation caused by the seasonal changes.
Since the line-focus is realized by concentrating the rays on a line, as long as the heat collecting pipe has a certain length, it allows to adjust the elevation every few days or even every season to collecting light instead of tracking the daily solar movement. The difficulty of manufacturing and cost of line-focus has a significant reduction compared with that of point-focus. Hence, even through the focus temperature is slightly lower than that of point-focus, it greatly attracts public attention and has many applications in developed countries.
The method of groove focus of solar energy has a simpler structure and lower cost than a tower-focus structure, and is easier to be used in tracking the sun by adjusting the solar elevation every few days or even longer depending on different seasons. The method needs only one repetition each year. Therefore, the groove focus becomes a more popular heat utilization method than tower-focus. However, most of the solar power towers described above are for study or test purpose, which have many disadvantages such as large investment, uneconomical, too large area and difficult maintenance, etc.
Solar power generation is definitely low-carbon in view of the environment protection, whilst the manufacturing of the products used for photovoltaic solar power, such as monocrystalline silicon, is actually a high-carbon industry. Even through the photovoltaic solar power is the most efficient way at present in the solar power utilization, which has approximately 15% photoelectric conversion efficiency, and even reaches 20% in the laboratory. The manufacturing of ultra-pure silicon requires a complicated process and a large power consumption, which accounts for over half of the total cost of manufacturing the solar cell. Thin-film solar cells made from amorphous silicon appeared in 1979 has a low consumption of silicon material, low cost and low weight, and is easy to be used. It can be combined with the roof of buildings to form an independent power supply of household. However, its photoelectric conversion efficiency is as low as about 10% in foreign countries and about 6%-8% in China. Moreover, thin-film solar cells are not stable and have a phenomenon of efficiency recession, and thus are not used for large solar power supply.
The thin-film solar cells developed recent years, such as cadmium sulfide, gallium arsenide, copper indium selenium etc. can reduce the cost of manufacturing monocrystalline silicon and the power consumption. However, the photoelectric conversion efficiency is between 6%-10% only and the production environmental cost is high (gallium is rare; arsenic is poison; the production investment is large; and the photoelectric conversion efficiency of cadmium sulfide is about 3.5%).